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The simplest of the three patterns is called blue-green deployment. In this pattern, we have two production environments: blue and green. At any time, only one of these is serving customer traffic--in figure 20, the green environment is live. Figure 20: Blue-green deployment patterns (Source: Humble and North, Continuous Delivery, 261.) To release a new version of our service, we deploy to the inactive environment where we can perform our testing without interrupting the user experience. When we are confident that everything is functioning as designed, we execute our release by directing traffic to the blue environment. Thus, blue becomes live and green becomes staging. Roll back is performed by sending customer traffic back to the green environment.